If you are in an rf-dense environment, where things like NOAA, 152mhz pagers, FM broadcast etc causes the front-end of your scanner to desense, then I'd heartily recommend using a duck from the aviation industry itself:
ICOM FA-BO2AR
This bnc mount duck is what is used on many of their commercial aviation line. The key is that it is narrowband for just aviation only.
When you test one of these against other antennas, marvel at the LOSS of broadcast, pager, NOAA and other out of band reception that you don't want when doing more serious portable aviation monitoring.